1. Field of the Invention
The present invention relates to a carrier that is used for latent electrostatic image development and is suitable for electrophotography, electrostatic recording, electrostatic printing and the like and a manufacturing method thereof, a developer using the carrier; and a process cartridge, image forming apparatus and image forming method using the developer.
2. Description of the Related Art
A dry development system used for electrophotography makes toner that has caused friction with a charging member electrostatically adhere to a latent electrostatic image to form a visible image. Such dry development systems include a one-component development system using toner as the main component and a two-component development system using glass beads, magnetic carriers, or carriers prepared by coating the surfaces of such materials with resin or the like, in combination with toner.
In a developer used in such a two-component development system, minute toner particles are held on relatively large core surfaces by an electric force produced by friction therebetween. When this developer is made close to a latent electrostatic image, owing to an electric field formed by the latent electrostatic image, the toner overcomes the binding force with the core, and transferred on the latent electrostatic image to develop this image. Then, the developer is repeatedly used while being replenished with the toner consumed by development. Therefore, during use over an extended period of time, the core must always frictionally charge the toner with a desired polarity and to a sufficient amount of charge. However, due to inter-particle collision, mechanical stirring of particles within a development device, or heat generation caused by these phenomena, a phenomenon called “spent toner” occurs where the toner is fusion-bonded to the core surface, and thereby charging characteristics of the core deteriorates with operating time. As a result, since background smearing of an image and toner scattering occur, it becomes necessary to replace the entire developer.
For preventing such a spent phenomenon, many proposals have been made for extending the lifespan of carriers by coating the core surface with a resin having a low surface energy, for example, a fluorine resin, a silicone resin, or the like. The proposed carriers include, for example, a carrier coated with a room-temperature curable silicone resin and a positive charge nitrogen resin (see Japanese Patent Application Laid-Open (JP-A) No. 55-127569), a carrier coated with a coating material containing at least one type of modified silicone resin (see JP-A No. 55-157751), a carrier having a coating layer containing a room-temperature curable silicone resin and a styrene-acryl resin (see JP-A No. 56-140358), a carrier for which the surface of a core particle is coated with two or more layers of silicone resin, with no adhesion provided between the adjacent layers (see JP-A No. 57-96355), a carrier for which a multilayered silicone resin is applied to the surface of a core particle (see JP-A No. 57-96356), a carrier whose surface is coated with a silicone resin containing silicon carbide (see JP-A No. 58-207054), a positively charged carrier coated with a material having a critical surface tension of 20 dyn/cm or less (see JP-A No. 61-110161), and a developer composed of carriers coated with a coating material containing fluorinated alkyl acrylate and a toner containing a chromium-containing azo dye (see JP-A No. 62-273576).
Recently, there has been a trend to reduce the particle diameter of toner for realizing a high image quality, and as a result, toner spent to the carrier easily occurs. In addition, by a conventional spray coating, it is difficult to uniformly moisten the carrier surface with a coating material, and therefore, it is difficult to manufacture a carrier having adhesion between the coating film and core and a uniform film thickness and film quality. Furthermore, in the case of a full-color toner, since a low-softening-point resin is used in order to obtain a sufficient color tone, in comparison with that of a black toner, the spent amount to the carrier is large, the amount of toner charge is lowered, and toner scattering and background smears occur. Thus, a full-color electrophotographic system has a problem that the image density of, in particular, a highlight is easily changed and a high image quality cannot be maintained when the amount of toner charge is lowered.
For improved carrier durability, it has also been proposed to provide a coating layer of resin matrix composed of low surface energy substance, in which fine particles and conductivity-imparting material are dispersed, to thereby control spent resistance, film strength, and electrical characteristics (see JP-A Nos. 09-319161, 09-269614, and 10-186731). However, in these proposals, since a dispersion prepared by adding fine particles to an organic solvent is applied by high-temperature spray coating, there is a problem that it is difficult to unify the amount of charge due to aggregation of the fine particles.
Conventional spray coating techniques also suffer from operational problems, such as regulation of volatile organic compounds (VOC) resulting from an organic solvent used for formation of a coating layer on the core surface, generation of liquid waste, and requirement of energy for drying.
In order to overcome these problems, for example, as manufacturing methods for carrier that involve an organic solvent-free dry powder process, manufacturing methods that involve use of a supercritical fluid have been proposed (see U.S. Pat. No. 5,514,512). However, this proposal discloses application of a coating resin, which has been polymerized in a supercritical fluid, by melting it in a heated state, and the resin material is an acrylic resin; thus the aforementioned toner spent problem has not yet been solved.
In addition, for improved carrier durability, as a carrier which is stable without deterioration in charging characteristics even when the carrier coating layer has worn down during continuous use, a carrier with a layered structure where silane coupling agents in the coating layer have a concentration gradient has been proposed (see Japanese Patent (JP-B) No. 3286849). However, since it is necessary to dissolve silane coupling agents having different concentrations in organic solvents and to subsequently apply the resultant solutions by spray coating sequentially, there is a problem that interfaces easily occur in the coated layers, the degree of film scraping increases, and it is difficult to control the layered structure.
Furthermore, in the manufacturing method for carrier using a supercritical fluid (see JP-A No. 2006-106208), a silicone resin is used for the coating material; however, its solubility to the supercritical fluid is insufficient, and a carrier is manufactured through a process in which a plasticized and microdispersed silicone resin is sprayed simultaneously with core for coating of the core surface. For this reason, it results in non-uniform coating thickness and unevenness in the coating layer, and therefore, it results in failure to obtain a coating thickness sufficient to ensure carrier durability.
Furthermore, in the fields of electrical appliances, automobile bodies, ship hulls, buildings, and the like, as a coat manufacturing method for forming a film without using an organic solvent for a base material having a complicated shape and a base material having a narrow gap and without applying a heating treatment, a method for applying a silicone resin or the like to a base material such as a stainless steel plate and an aluminum plate by making the base material contact with the silicone resin or the like in the presence of a supercritical fluid within a container and then reducing the pressure of the container has been proposed (see JP-A No. 2004-833).
However, it is considerably difficult to conceive application of techniques of such different fields to manufacturing of a carrier coating layer, and even if this could be conceived, the solubility of the silicone resin to the supercritical fluid is low, the resultant film has a small thickness, and thus carrier durability is not satisfactory even when the film is used for a carrier coating layer.
As described above, a carrier that has a coating layer on the core surface, is high in adhesion between the core and coating layer, and has a coating layer with a uniform thickness has not yet been provided, and furthermore, from the perspective of global environmental loads and resource savings as well, conventional carrier manufacturing methods and carriers still have many problems that require solutions under the present circumstances.